fix: enforce non-negative policy priority

[amavashev]

Summary

• add request-model validation for non-negative policy priorities on create and update
• prove negative requests return 400 INVALID_REQUEST before persistence and zero remains valid
• advance deterministic contract validation and documentation to governance spec 0.1.25.42

Validation

• focused PolicyModelTest + PolicyControllerTest: 43/43
• clean full non-integration Maven verification: 1,855/1,855
• JaCoCo line / branch: model 97.85% / 98.44%; data 97.48% / 95.35%; API 97.47% / 95.05%
• GitHub Unit & Contract and Integration jobs: green
• git diff --check

Ecosystem links

• contract dependency: fix(governance): require non-negative policy priority cycles-protocol#141 at 402307a88906e9fd090159e5ccf2d0036e6aec83
• dashboard compatibility layer: fix: enforce policy priority minimum cycles-dashboard#273